Conventionally, as a valve core of this type, known is one including a first core cylinder body having a male screw on its outer peripheral surface and a second core cylinder body having a seal portion on its outer peripheral surface which are rotatably connected to each other. On the other hand, a reduced diameter portion is provided on a deeper side than a female screw in a core mounting hole. When the valve core is screwed and mounted in the core mounting hole, only the first core cylinder body is rotated in a state where the seal portion of the second core cylinder body is in close contact with the reduced diameter portion, whereby damage of the seal portion can be prevented during the screw mounting (for example, refer to Patent Literature 1).